Actuator apparatus

ABSTRACT

An actuator for converting electrical energy into linear mechanical movement comprises a chamber containing an expandable material and a positive temperature coefficient (PTC) of resistance heater located in thermal communication with the chamber. A first piston transmits force from the expandable material to an external load against the force of a first return spring. The first piston moves out of a casing from a retracted position at a first stop to a protracted position at a second stop at which point continued expansion of the expandable material in one embodiment causes a second piston to separate the chamber from the heat source against the bias of a second return spring. As the chamber separates the thermal resistance between the heat source and the chamber greatly increases until equilibrium is obtained between heat input to the expandable medium and pressure exerted by the medium. Another embodiment particularly useful for high ambient temperatures and for a longer time period before the actuator returns to its at rest position after deenergization of the heat source employs a chamber fixed in the casing but having a secondary piston which moves to expand the chamber volume to match the volume increase of the expandable material. Several variations are shown for mounting the actuator to a support.

United States Patent 1 Marcoux et al.

[ Jim-1,1974

l 54 l ACTUATOR APPARATUS [75] Inventors: Leo Marcoux, Rehoboth; Peter G.

Berg, Norton, both of Mass.

[73] Assignee: Texas Instruments incorporated,

Dallas, Tex.

[22] Filed: May 1, 1972 [21] Appl. No.: 248,942

Primary ExaminerEdgar W. Geoghegan Assistant ExaminerAllen M. Ostrager Attorney-Harold Levine et al.

[57] ABSTRACT An actuator for converting electrical energy into linear mechanical movement comprises a chamber containing an expandable material and a positive temperature coefficient (PTC) of resistance heater located in thermal communication with the chamber. A first piston transmits force from the expandable material to an external load against the force of a first return spring. The first piston moves out of a casing from a retracted position at a first stop to a protracted position at a second stop at which point continued expansion of the expandable material in one embodiment causes a second piston to separate the chamber from the heat source against the bias of a second return spring. As the chamber separates the thermal resistance between the heat source and the chamber greatly increases until equilibrium is obtained between heat input to the expandable medium and pressure exerted by the medium.

Another embodiment particularly useful for high ambient temperatures and for a longer time period before the actuator returns to its at rest position after Several variations are actuator to a support.

shown for mounting the 22 Claims, 14 Drawing Figures PAIENTEDJAH H974 3.782.121

WET 10E 6 PAIENTEDJAM H974 3,782,121

SRKET 50E 6 PMENTEDJAH 1 am I 3,782,121

I sum 60? s 1 ACTUATOR APPARATUS This invention relates generally to thermal actuators and more particularly to a thermal actuator for converting electrical energization into mechanical movement.

In copending, coassigned application, Ser. No. 168,657 filed Aug. 3, 1971 a thermal actuator is disclosed and claimed in which a self-regulating heat generating element is disposed in thermal communication with a chamber containing a thermally expandable medium. Electrical energization of the heat generating element causes the thermally expandable medium to expand which in turn causes linear movement of a piston mounted in force receiving relation to the medium. Such an actuator has many advantages over prior art devices. For example, use of the self-regulating heat generating element provides a device in which overheating is avoided even though substantially constant electrical energization may be employed. Further, the

necessity for external devices for limiting electrical en-' ergization is eliminated. This actuator, as well as other prior art actuators, is calibrated to produce a desired stroke, however, this calibrator is subject to change over the life of the device due to loss of some of the expandable medium from the chamber or by a change in volume of the chambers should it be deformed by an accidental blow. Such actuators are also subject to damage should the piston be prevented from movement by some static, external force. Further, the maxi mum temperature of the heat generating device must be chosen low enough that deterioration ofthe expandable medium is not effectedl While these characteristics do not serve as limitations for many uses of thermal actuators there are certain situations where they do become important. One such example is in a dishwashing machine in which the actuator is used to cause dispensing of soap or'arinsing agent. Adverse operating conditions such as humidity and dirt can create linkage sticking which could cause deformation of the chamber containing the expandable medium. That is, the medium will continue to expand even if the piston is stuck and will deform the chamber containing it. Another example would be an applicationin which it is critical to maintain an accurately determined, given stroke throughout the life of the device.

Briefly these prior art limitations are obviatedby the present invention in which a thermal actuator having a force applying piston is providedin which the stroke of the piston is accurately determined by mechanical stops. When the piston is fully extended, in one embodiment continued expansion of the expandable medium causes the chamber containing the expandable medium to separate from the heat source against the bias of a second return spring thereby altering thermal communication from the heat source to the medium to maintain the pressure level within the desired limits. Faster response time of the actuator can be obtained since a higher temperature heat source can be employed due to this separation. In an alternative embodiment the location of the chamber is fixed within the casing but asecond piston is driven to expand the available volume of the chamber.

It is an object of the present invention to provide an improved thermal actuator which converts electrical energy into linear mechanical movement of an accurately predetermined stroke. Another object of the invention is the provision of an actuator whose stroke is adjustable but will not vary during its useful life. Yet another object is the provision of an improved thermal actuator which will not be damaged by an immovable external static load. Another object is the provision of a thermal actuator which has shorter warmup time without a concomitant longer cool down time.

Other objects and features of the invention may be more readily understood from the following detailed description and appended claims when read in conjunction with the accompanying drawings.

In the accompanying drawings:

FIG. 1 is a front elevation of a thermal actuator made in accordance with the invention;

FIG. 2 is a top view of the actuator shown in FIG. 1; FIG. 3 is a bottom view of the actuator;

FIG. 4 is a cross section taken on lines 44 of FIG.

FIG. 5 is a graph of log resistance versus temperature for a heating element used in the actuator;

FIG. 6 is a graph of volumetric expansion versus temperature for a thermally expandable material useful in the actuator of the present invention;

FIG. 7 is a simplified cross section similar to FIG. 4 showing the actuator in the unactuated position;

FIG. 8 is a view similar to FIG. 7 showing an intermediate state of actuation in which the force applying piston has just reached its full stroke;

FIG. 9 is a view similar to FIGS. 7 and 8 but showing the device fully actuated;

FIG. 10 is a view similar to FIG. 7 showing means to adjust the length of stroke of the actuator piston;

FIG. 11 is a view similar to FIG. 8 showing an alternative embodiment in an intermediate state of actuation in which the force applying piston has just reached its full stroke;

FIG. 12 is a view similar to FIG. 11 showing the actuator of the alternative embodiment in its fully actuated state;

FIG. 13 shows the actuator mounted on a support; and

FIG. 14 shows an alternative mounting for the actuator.

Similar reference characters indicate corresponding parts throughout the several views of the drawings. Dimensions of certain of the parts, as shown in the drawings, have been modified or exaggerated for the purpose of clarity of illustration.

Referring to the drawings, particularly to FIGS. 1-4 a thermal actuator is designated generally by reference numeral 10. An outer casing of electrically insulative material such as electrically insulative phenolic resin is shown formed of two identical halves 12, 14 split longitudinally and define a generally cylindrical shaped cavity 16 therein. Apertures l8 and 20 are provided in each casing half which receive rivets (not shown) to fix the halves 12, 14 together. A pin 22 is formed in the casing half for insertion in a mating depression 24 of a like casing half to insure proper alignment of the two halves.

Contained in cavity 16 is a piston member 26, generally cylindrical in configuration, which conveniently may be molded out of phenolic material. Piston member 26 is formed with an integral outwardly extending radial flange 28 on one end thereof the periphery of which is closely received in cavity 16. A coil spring 30 is placed about piston member 26 between flange 28 and top wall portion 32 of the casing so that piston member 26 is adapted to slide within the cavity against the bias of spring 30. Piston member 26 is formed with a reduced diameter rod portion 34 which forms an annular shoulder 36 with the main body of member 26 which serves to limit outward movement of member 26 by abutment with top wall 32 of the casing. Aperture 38 in top wall portion 32 slidingly receives reduced diameter portion 34. Also formed in the casing is an annular seat 40 against which bottom surface 44 of flange 28 is biased by spring 30. Thus it will be seen that piston member 26 is biased by spring 30 into an unactuated retracted position with flange 28 abutting annular surface 40 which functions as a stop while outward movement of the piston member 26 is limited by annular shoulder 36 abutting wall portion 32 which functions as a second stop.

Piston member 26 is provided with a bore 42 communicating with bottom surface 44 and is formed with an integrally formed downwardly extending rod 46. The depth of bore 42 is chosen so that it exceeds the length of the piston stroke.

A chamber 48, generally cylindrical, is formed of a thermally conductive material such as brass and has a first body portion 50 joined to a second open ended sleeve portion 52 by a truncated portion 54. Sleeve portion 52 extends into bore 42 of piston member 26 and preferably fits closely to the side wall of bore 42 to serve as guiding means to keep the parts aligned and prevent any tendency of wobbling. An outwardly extending flange 56 is formed on the free end of body portion 50 to facilitate crimping attachment of base 58 as shown at 60. Base 50 is provided with an open ended tubular sleeve 62 which extends inwardly into the chamber. A thermally expandable medium 64 of a type to be more fully described below is contained within chamber 48. A first floating piston head 66 is slidingly received within sleeve 52 while a second floating piston head 68 is slidingly received in sleeve 62. Floating piston heads 66, 68 are provided with sea] seating grooves 70, 72 respectively which receive therein suitable O- ring seals 74, 76 respectively. Seals 74, 76 are formed of relatively flexible rubber like material capable of continuous high temperature operation such as a fluoroelastomer material.

A second coil return spring 78 is received between crimped flange portion 60 of body 50 and a shoulder 80 formed in the casing which spring biases chamber 48 toward the bottom of the casing against a force receiving, thermally conductive plate 82.

Terminals 84, 86 extend through apertures provided in the bottom wall 88 of the casing. Terminal 84 is preferably formed with a laterally extending circular plate 90 seated on shoulder 98 formed in the casing and to which is bonded a wafer 92 of material having a positive temperature coefficient of resistance (PTC). The wafer is provided with metalized contact surfaces on both opposite faces, one face may be either soldered to plate 90 or cemented thereto as by electrically conductive silver epoxy. Terminal 86 is formed with a laterally extending platform 94. Electrically conductive terminal spring 96 extends between the second face of wafer 92 and platform 94.

Wafer 92 is preferably composed of material having a positive temperature coefficient of resistance (PTC) with a steep slope at temperatures above an anomaly so that upon electrical energization it will self regulate at a chosen temperature. An example of suitable material is semiconducting barium titanate ceramic doped with lanthanum such as Ba La TiO Although other anomaly temperatures may be employed the log resistance versus temperature curve 100 of a PTC wafer useful in the invention is shown in FIG. 5. It will be noted that at temperatures below an anomaly temperature of approximately 120C. the resistance is essentially constant, however, above the anomaly temperature the resistance abruptly increases by several orders of magnitude. When a suitable voltage is used to energize the wafer the temperature thereof rapidly increases until the anomaly point is reached when the resistance abruptly increases and limits the current and power input into the wafer. The input power is reduced until the wafer reaches thermal equilibrium at an operating temperature of approximately 120C. designated by T, on curve 100. Although use of standard resistance heaters is within the purview of the invention a PTC heater is preferred because it is particularly well suited for use with thermal actuators. That is, since the amount of expansion of the expandable medium is directly dependent upon temperature of the medium it is advantageous to use heaters having a chosen constant predetermined operating temperature, and steeply sloped PTC heaters have this characteristic. For instance, variations in input voltage have a negligible effect on the operating temperature of the PTC wafer. An increase in input voltage initially increases input power to the wafer causing a small increase in operating temperature which causes a larger increase in resistance which in turn drives the input power level back toward its original operating point. A decrease in input voltage initially decreases input power causing a small decrease in operating temperature which causes a larger decrease in resistance which in turn drives the input power level back up to its operating point.

Variations in ambient temperature do not affect the operating point of the PTC wafer since the anomaly temperature is well above the ambient temperature. Since the PTC wafer converts electrical energy to heat by PR heating either AC or DC power sources can be employed. No radio frequency interference is transmitted since there are no contacts or other moving parts.

Many materials are available for use as the thermally expandable medium. FIG. 6 shows the volume expansion versus temperature curve 102 for one such material which is a processed paraffin wax having a narrow molecular weight distribution. Such material has a volumetric expansion coefficient which is much greater in the solid to liquid phase change range than either the solid or the liquid expansion coefficient. The wax maintains its expansion properties indefinitely and they are virtually independent of pressure. The particular material chosen for medium 64 is such that it has a phase change temperature range above the highest ambient expected during operation of the actuator since it is desired that piston movement take place during the high expansion phase change of the medium. The medium represented in FIG. 6 has a phase change range from to C. and is particularly useful in the invention with a PTC wafer having an anomaly temperature higher than the range, e.g. C. as will be explained below.

Operation of actuator 10 will be explained with particular reference to FIGS. 7-9 which are somewhat simplified facilitate understanding of the operation.

FIG. 7 depicts actuator 10 in the unenergized or unactuated condition as shown in FIG. 4. Upon energization of heat source 92 by impressing a voltage thereacross through terminals 84, 86 the temperature of the PT C wafer quickly reaches its operating point T, (FIG. 5) and heat is conducted to the expandable medium 64 through brass chamber 48. It will be noted that sleeve 62 which projects into the chamber enhances heat transfer from the heat source to the medium 64. When the temperature of the medium reaches 80C. the phase change begins to occur and forces piston member 26 outwardly against the bias of spring 30 until stop surface 36 of piston member 26 abuts wall 32 of the chamber thereby limiting the piston stroke, as seen in FIG. 8. Medium 64 continues to expand however and causes chamber 48 to separate from heat source 92 against the bias of spring 78, as seen in FIG. 9. In order for this to occur the effective piston area of floating piston 68 is chosen so that it is larger than the effective piston area of floating piston 66. Thus there is a net force on truncated portion 54 causing the chamber to move away from the heat source and plate 82 (FIG. 4) dependent upon the difference in the two areas. Once the chamber separates from the heat source heat transfer to the medium is reduced even though the PTC wafer is still at its operating temperature. Thus a balance is obtained between pressure in the chamber and heat transferred to the medium. The further the chamber separates from the heat source the less the amount of heat transferred to the medium. Although the separation shown in FIG. 9 is exaggerated for purposes of illustration in actual operation it has been found that this balance typically occurs with a separation (air gap) of 30 to 40 thousandths of an inch. The system is designed to reach equilibrium at a decreased heat input level to medium 64 with approximately half of the phase change expansion used to actuate piston member 26. The isostatic pressure which triggers movement of chamber 48 is dependent only on the relative areas of the first and secondpistons 66, 68 and the loading of return spring 78.

When the actuator is deenergized the chamber 48 moves back into contact with support plate 82 and piston member 26 then moves back to the retracted position as shown in FIG. 7 due to the return force of spring 30.

Thus it will be seen that if a small amount of medium 64 is lost during cycling the medium will heat up to a slightly higher temperature before piston member 26 completes its stroke and the pressure buildup begins. The external load capabilities and the length of piston travelwill not change with loss of expandable medium from chamber 48 until the reserve supply is depleted. That is, initially actuator 10 makes use of the lower half of the solid-liquid phase change expansion (FIG. 6) for actuation while near the end of its useful life it makes use of the upper half of the solid-liquid phase change and operates at about 7C. higher in temperature.

Another advantage obtained by use of a primary, secondary piston system is the ability of the actuator to protect itself against pressure damage due to static external loads in excess of design limits. That is, if piston member 26 were to meet a static immovable force during it s stroke the pressure in chamber 48 would increase causing chamber 48 to separate reaching equilibrium at a stall mode .with reduced heat input. If the excess static force were removed piston member 26 would continue its outward movement and complete its stroke. This could be a common or normal occurrence in certain applications such as in automatic ejection of ice cubes from a tray upon sufficient heating of separator elements.

Separation of chamber 48 from the heat source also permits use of a PTC wafer with a higher anomaly point than would otherwise be possible without driving the expandable medium to excessive temperature since the heat input to the medium is reduced by the separation limiting the temperature rise of the medium.

In some uses it may be desirable to provide a shorter piston stroke. As seen in FIG. 10 this can be conveniently provided by adding one or more annular spacer elements 104 so that any desired stroke can be provided in a given package configuration up to the total equal to the distance between stop 36 and wall portion 32 when piston member is bottomed against seat 40 of the casing.

Another embodiment is shown in FIGS. 11 and 12, FIG. 11 showing an actuator just after piston member 26 has reached its full protracted position and FIG. 12 showing the actuator fully actuated and in equilibrium. This embodiment is particularly useful when it is desired to have a longer cool down time after deenergization of the actuator. For instance, when used for a locking mechanism for a high temperature over such as a pyrolytic self-cleaning oven when a positive time delay is desired to permit sufficient cooling of the oven interior before opening of the oven can be effected even if there is a loss of electrical power. As seen in FIG. 11 actuator has a modified expandable medium chamber 112. Chamber 112 is fixed in the casing by any appropriate means and has a floating piston head 114 which is biased against seat 116 by spring 118. The other components of this actuator are identical to those of actuator 10 and operation is the same except that instead of effecting an equilibrium by means of reducing heat transfer by separation of the chamber from the heat source the volume of the chamber is allowed to increase by depression of piston head 114 against the bias of return spring 118, as seen in FIG. 12. Thus the temperature of the medium will be maintained at a higher level even after deenergization of the actuator due to its close thermal coupling with the heat source during the entire cycle.

This embodiment is also particularly useful when the actuator is to be used in exceptionally high ambient temperature environments, as in an engine compartment of an automobile. Depression of piston head 114 increases the volume available for the expanding wax significantly more than in the previous embodiment in which the volume increase in sleeve portion piston 62 is partly offset by the volume decrease in sleeve more volume 52. Further, if it is desired'to provide even morevolume available for the expanding wax the diameter of the sleeve 52 and hence also its bore can be increased.

The actuator of the invention can be mounted in several convenient ways. For example, FIG. 13 shows actuator 10 on a support 130. A plurality of grooves 132 separated by lands 134 are provided around the periphery of the casing adapted to receive crescent rings 136 to lock the actuator in place. Thus'the distance the actuator projects through the support may be adjusted by locating the actuator so that any land 134 is aligned with support 130. Alternatively actuator 10 may be received in an aperture of bracket 138 as seen in FIG. 14 with shoulder 140 of the casing biased against bracket 138 and push ring 142 locking the actuator in place.

As many changes could be made in the above constructions without departure from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

It is to be understood that the invention is not limited in its application to the details of constructionand arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

We claim:

1. Actuator apparatus comprising a hollow casing defining a cavity therein;

a chamber having first and second force output means received in the cavity, a thermally expandable medium contained in the chamber, energizable heat source means mounted in thermal communication with the thermally expandable medium, a first piston member slidably mounted in the casing and movable relative to the casing from a retracted position to a protracted position, means coupling the first piston member to the first force output means of the chamber to cause movement of the first piston member from the retracted position to the protracted position upon sufficient expansion of the medium, a second piston member mounted in the casing for movement relative to the chamber, and means coupling the second piston member to the second force output means of the chamber to cause movement of the second piston relative to the chamber.

2. An actuator according to claim 1 in which the second force output means of the chamber is formed by means defining a bore, the second piston member slidably mounted in the bore, and means biasing the piston member toward the expandable medium, the biasing means placing a given force on the second piston member which force is greater than that required to move the first piston member from the retracted to the protracted position whereby continued expansion of the medium after the first piston has moved to the protracted position will cause the second piston member to move in a direction away from the medium to increase the effective volume of the chamber.

3. An actuator according to claim 1 in which the second force output means of the chamber is formed by means defining a bore, the second piston member mounted within the bore, means limiting movement of the second piston member away from the medium, the chamber slidably mounted within the cavity, means biasing the chamber in a direction generally parallel to the axis of the bore, the biasing means placing a given force on the chamber in excess of a predetermined force used for moving the first piston member from the retracted to the protracted position whereby continued expansion of the medium after the first piston has moved to the protracted position will cause the chamber to move relative to the casing.

4. An actuator according to claim 3 in which the heat source is mounted in the casing and the biasing means places a force on the chamber tending to maintain the chamber in close thermal communication with the heat source, movement of the chamber caused by continued expansion of the medium after the first piston member has moved to the protracted position will reduce heat received by the medium within the chamber and an equilibrium will be effected between heat input to the medium and pressure exerted by the medium.

5. An actuator according to claim 4 in which the heat source comprises a wafer of material having a positive temperature coefficient of resistance.

6. An actuator according to claim 5 in which the expandable medium has a solid to liquid phase change temperature range and the heat source wafer has a anomaly temperature above which the wafer has a steeply sloped positive temperature coefficient of resistance, the anomaly temperature being higher than the solid to liquid phase change temperature range of the expandable medium.

7. An actuator comprising a casing defining a cavity therein, electrical energizing means mounted in the casing, a chamber having first and second force output means mounted in the cavity in energy receiving relationship with the electrical energy means, an expandable medium contained in the chamber, a first piston member slidingly mounted in the casing between a retracted and a protracted position, the piston member having a force receiving surface located in force receiving relation with the first force output means of the chamber, a second piston means mounted in the casing, the second piston means having a force receiving surface located in force receiving relation with the second force output means of the chamber, the second force receiving surface larger than the first force receiving surface, and means biasing the second piston means in a direction opposed to the force exerted by the expandable medium, the biasing means exerting a given force on the second piston means in excess of a predetermined force used for moving the first piston member from the retracted to the protracted position whereby the expandable medium expands upon electrical energization causing the first piston member to move from the retracted position to the protracted position, continued expansion of the expandable medium causes relative movement of the second piston means until an equilibrium is reached between power input and pressure exerted by the expandable medium.

8. Actuator apparatus for translating electrical energy into linear mechanical movement comprising:

a casing defining a cavity therein, the casing having a top wall with an aperture extending therethrough into the cavity,

a chamber formed of thermally conductive material having first and second bores communicating with the chamber, the chamber received in the casing,

a thermally expandable medium contained in the chamber,

an electrical resistance heater received in the casing in thermal communication with the chamber,

a first piston member slidably received in the casing cavity movable between a retracted position and a protracted position, the piston member having a first force receiving surface positioned within the first bore of the chamber, first return spring means positioned in the casing cavity and biasing the first piston member toward its retracted position,

means to electrically energize the electrical resistance heater,

second piston means including a member having a second force receiving surface received in the second bore of the chamber, the second force receiving surface movable relative to the chamber, and

second return spring means preventing relative movement of the second force receiving surface and the chamber until a predetermined force is exerted on the first piston member.

9. An actuator according to claim 8 in which the location of the chamber is fixed within the casing cavity, the second return spring biasing the member of the second piston means inwardly toward the interior of the chamber, and means limiting inward movement of the member of the second piston means.

10. An actuator according to claim 8 in which the chamber has a generally cylindrical body portion having two opposite ends a first elongated sleeve portion of reduced diameter and an intermediate truncated portion joining one end of the body portion with the first sleeve portion, and a bottom wall having an aperture therein located at the second end of the body portion, the bottom wall formed with a second elongated sleeve in communication with the aperture in the bottom wall and extending into the interior of the chamber, the first sleeve defining the first bore and the second sleeve defining the second bore.

1 1. An actuator according to claim 10in which a first floating piston member is received in the first bore, a second floating piston member is received in the second bore, the floating piston members each having a groove formed therein and elastomeric sealing means received in each respective groove tightly fitting between the piston members and the surfaces defining the respective bores.

12. An actuator according to claim 10 in which the chamber is movably received in the casing cavity and is formed with an outwardly extending radial flange, a shelf is formed in the casing, the second return spring is a helical spring extending between the radial flange of the chamber and the shelf of the casing, and means is provided to limit outward movement of the second piston member.

13. An actuator according to claim 12 in which the first and second bores are circular and the diameter of the second bore exceeds the diameter of the first bore.

14. An actuator according to claim 8 in which the electrical resistance heater comprises a wafer of material having a positive temperature coefficient of resistance.

15. An actuator according to claim 14 in which the wafer is comprised of barium titanate doped with a rare earth, the wafer having two metallized opposite faces.

16. An actuator according to claim 15 including first and second terminal means extending from without the casing into the casing cavity, the first terminal having a surface extending laterally across the cavity, the second terminal having a laterally extending platform within the cavity, one face of the wafer bonded to the laterally extending surface of the first terminal and an electrically conductive spring biased into electrical contact between the other face of the wafer and the platform of the second terminal.

17. An actuator according to claim 16 in which the chamber is formed with a bottom wall which is located adjacent the laterally extending surface of the first terminal in thermal communication therewith.

18. An actuator according to claim 17 in which a heat conductive force distributing plate is placed between the laterally extending surface of the first terminal and the bottom wall of the chamber.

19. An actuator according to claim 8 in which the first piston member is formed with a rod portion which extends through the aperture formed in the top wall of the casing, a projecting surface is formed on the piston member and is adapted to abut an interior stop surface of the top wall to limit outward movement of the piston member, a stop shelf is formed in the casing cavity, the piston is formed with a surface adapted to abut the stop shelf to limit inward movement of the piston member whereby the piston movement is accurately determined by the distance between the two stops.

20. An actuator according to claim 19 including a spacer mounted on the rod portion of the first piston member to shorten the piston movement.

21. An actuator according to claim 8 in which the casing is generally cylindrical and is formed with a plurality of lands and grooves, the grooves adapted to receive snap rings for mounting of the actuator in desired adjusted position.

22. Actuator apparatus comprising a hollow casing defining a cavity therein;

a chamber having a force output means received in the cavity, a thermally expandable medium contained in the chamber, energizable heat source means mounted in thermal communication with the thermally expandable medium, a piston member slidably mounted in the casing and movable relative to the casing from a retracted position to a protracted position, means coupling the piston member to the force output means of the chamber to cause movement of the piston member from the retracted position to the protracted position upon sufficient expansion of the medium, and means for moving the chamber relative the heat source means to decrease heat input to the medium when the piston moves to the protracted position. 

1. Actuator apparatus comprising a hollow casing defining a cavity therein; a chamber having first and second force output means received in the cavity, a thermally expandable medium contained in the chamber, energizable heat source means mounted in thermal communication with the thermally expandable medium, a first piston member slidably mounted in the casing and movable relative to the cAsing from a retracted position to a protracted position, means coupling the first piston member to the first force output means of the chamber to cause movement of the first piston member from the retracted position to the protracted position upon sufficient expansion of the medium, a second piston member mounted in the casing for movement relative to the chamber, and means coupling the second piston member to the second force output means of the chamber to cause movement of the second piston relative to the chamber.
 2. An actuator according to claim 1 in which the second force output means of the chamber is formed by means defining a bore, the second piston member slidably mounted in the bore, and means biasing the piston member toward the expandable medium, the biasing means placing a given force on the second piston member which force is greater than that required to move the first piston member from the retracted to the protracted position whereby continued expansion of the medium after the first piston has moved to the protracted position will cause the second piston member to move in a direction away from the medium to increase the effective volume of the chamber.
 3. An actuator according to claim 1 in which the second force output means of the chamber is formed by means defining a bore, the second piston member mounted within the bore, means limiting movement of the second piston member away from the medium, the chamber slidably mounted within the cavity, means biasing the chamber in a direction generally parallel to the axis of the bore, the biasing means placing a given force on the chamber in excess of a predetermined force used for moving the first piston member from the retracted to the protracted position whereby continued expansion of the medium after the first piston has moved to the protracted position will cause the chamber to move relative to the casing.
 4. An actuator according to claim 3 in which the heat source is mounted in the casing and the biasing means places a force on the chamber tending to maintain the chamber in close thermal communication with the heat source, movement of the chamber caused by continued expansion of the medium after the first piston member has moved to the protracted position will reduce heat received by the medium within the chamber and an equilibrium will be effected between heat input to the medium and pressure exerted by the medium.
 5. An actuator according to claim 4 in which the heat source comprises a wafer of material having a positive temperature coefficient of resistance.
 6. An actuator according to claim 5 in which the expandable medium has a solid to liquid phase change temperature range and the heat source wafer has a anomaly temperature above which the wafer has a steeply sloped positive temperature coefficient of resistance, the anomaly temperature being higher than the solid to liquid phase change temperature range of the expandable medium.
 7. An actuator comprising a casing defining a cavity therein, electrical energizing means mounted in the casing, a chamber having first and second force output means mounted in the cavity in energy receiving relationship with the electrical energy means, an expandable medium contained in the chamber, a first piston member slidingly mounted in the casing between a retracted and a protracted position, the piston member having a force receiving surface located in force receiving relation with the first force output means of the chamber, a second piston means mounted in the casing, the second piston means having a force receiving surface located in force receiving relation with the second force output means of the chamber, the second force receiving surface larger than the first force receiving surface, and means biasing the second piston means in a direction opposed to the force exerted by the expandable medium, the biasing means exerting a given force on the second piston means in excess of a predetermined force used for moving the first piston member from thE retracted to the protracted position whereby the expandable medium expands upon electrical energization causing the first piston member to move from the retracted position to the protracted position, continued expansion of the expandable medium causes relative movement of the second piston means until an equilibrium is reached between power input and pressure exerted by the expandable medium.
 8. Actuator apparatus for translating electrical energy into linear mechanical movement comprising: a casing defining a cavity therein, the casing having a top wall with an aperture extending therethrough into the cavity, a chamber formed of thermally conductive material having first and second bores communicating with the chamber, the chamber received in the casing, a thermally expandable medium contained in the chamber, an electrical resistance heater received in the casing in thermal communication with the chamber, a first piston member slidably received in the casing cavity movable between a retracted position and a protracted position, the piston member having a first force receiving surface positioned within the first bore of the chamber, first return spring means positioned in the casing cavity and biasing the first piston member toward its retracted position, means to electrically energize the electrical resistance heater, second piston means including a member having a second force receiving surface received in the second bore of the chamber, the second force receiving surface movable relative to the chamber, and second return spring means preventing relative movement of the second force receiving surface and the chamber until a predetermined force is exerted on the first piston member.
 9. An actuator according to claim 8 in which the location of the chamber is fixed within the casing cavity, the second return spring biasing the member of the second piston means inwardly toward the interior of the chamber, and means limiting inward movement of the member of the second piston means.
 10. An actuator according to claim 8 in which the chamber has a generally cylindrical body portion having two opposite ends, a first elongated sleeve portion of reduced diameter and an intermediate truncated portion joining one end of the body portion with the first sleeve portion, and a bottom wall having an aperture therein located at the second end of the body portion, the bottom wall formed with a second elongated sleeve in communication with the aperture in the bottom wall and extending into the interior of the chamber, the first sleeve defining the first bore and the second sleeve defining the second bore.
 11. An actuator according to claim 10 in which a first floating piston member is received in the first bore, a second floating piston member is received in the second bore, the floating piston members each having a groove formed therein and elastomeric sealing means received in each respective groove tightly fitting between the piston members and the surfaces defining the respective bores.
 12. An actuator according to claim 10 in which the chamber is movably received in the casing cavity and is formed with an outwardly extending radial flange, a shelf is formed in the casing, the second return spring is a helical spring extending between the radial flange of the chamber and the shelf of the casing, and means is provided to limit outward movement of the second piston member.
 13. An actuator according to claim 12 in which the first and second bores are circular and the diameter of the second bore exceeds the diameter of the first bore.
 14. An actuator according to claim 8 in which the electrical resistance heater comprises a wafer of material having a positive temperature coefficient of resistance.
 15. An actuator according to claim 14 in which the wafer is comprised of barium titanate doped with a rare earth, the wafer having two metallized opposite faces.
 16. An actuator according to claim 15 including first and second termiNal means extending from without the casing into the casing cavity, the first terminal having a surface extending laterally across the cavity, the second terminal having a laterally extending platform within the cavity, one face of the wafer bonded to the laterally extending surface of the first terminal and an electrically conductive spring biased into electrical contact between the other face of the wafer and the platform of the second terminal.
 17. An actuator according to claim 16 in which the chamber is formed with a bottom wall which is located adjacent the laterally extending surface of the first terminal in thermal communication therewith.
 18. An actuator according to claim 17 in which a heat conductive force distributing plate is placed between the laterally extending surface of the first terminal and the bottom wall of the chamber.
 19. An actuator according to claim 8 in which the first piston member is formed with a rod portion which extends through the aperture formed in the top wall of the casing, a projecting surface is formed on the piston member and is adapted to abut an interior stop surface of the top wall to limit outward movement of the piston member, a stop shelf is formed in the casing cavity, the piston is formed with a surface adapted to abut the stop shelf to limit inward movement of the piston member whereby the piston movement is accurately determined by the distance between the two stops.
 20. An actuator according to claim 19 including a spacer mounted on the rod portion of the first piston member to shorten the piston movement.
 21. An actuator according to claim 8 in which the casing is generally cylindrical and is formed with a plurality of lands and grooves, the grooves adapted to receive snap rings for mounting of the actuator in desired adjusted position.
 22. Actuator apparatus comprising a hollow casing defining a cavity therein; a chamber having a force output means received in the cavity, a thermally expandable medium contained in the chamber, energizable heat source means mounted in thermal communication with the thermally expandable medium, a piston member slidably mounted in the casing and movable relative to the casing from a retracted position to a protracted position, means coupling the piston member to the force output means of the chamber to cause movement of the piston member from the retracted position to the protracted position upon sufficient expansion of the medium, and means for moving the chamber relative the heat source means to decrease heat input to the medium when the piston moves to the protracted position. 